Manifold vacuum is actually a port that is below the throttle plate with the throttle closed. Port vacuum is a port that is above the throttle plate with the throttle closed. There is vacuum below the throttle plate and as the throttle is opened the port vacuum port is uncovered to allow vacuum at that port. Vacuum will vary with engine load and throttle position. There should be no vacuum at the port vacuum source at idle.

ahh i c i c, i realize vacuum changes with the position of the throttle plates i just didn't know the difference between the two. so you only have vacuum on port side when u start opening throttle plates because the ports are opened up?>

That is not true, vacuum varies. Manifold vacuum is greater than port with the throttle closed, but will decrease with the engine under load and be at zero at WOT. Port vacuum will also decrease under load or WOT, but will depend on how far up in the throttle bore the opening is. The closer to the venturi the vacuum will be greater. Port vacuum right at the venturi will become greater with throttle opening, but that is actualy called venturi vacuum. Part throttle going up a hill under good load port vacuum will probably be zero or close to that. That is the same as with manifold vacuum.

I copied this from an article that lots of people tend to use when swapping the carter to an mc2100.. I didn't write it, I take no credit!

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Understanding Types of Vacuum

YOU MUST UNDERSTAND THIS!!!!!!!

There are two types of vacuum pull that you will need to understand. They are “ported” vacuum and “manifold” vacuum. It is EXTREMELY important to know what type of vacuum source each port is pulling, and to know and understand which vacuum source is needed for each device. Example: Your distributor advance needs ported vacuum. The ported vacuum pull advances your distributor timing as you accelerate. If by chance you tie into manifold vacuum by mistake (and properly adjust your timing) you will actually retard your timing as you accelerate. This is BAD by the way and you will be very sluggish off the line and at WOT.

Ported Vacuum

Ported vacuum is drawn in when the throttle is opened up. This draw is increased as more air is sucked into your air through cleaner and down through the venturis. Ported vacuum is highest at W.O.T. and lowest (or non-existent) at idle. You will have almost no vacuum pull at idle with ported vacuum unless you have moderately high idle speeds..

Manifold Vacuum

Manifold vacuum is drawn when your throttle is closed or at idle to very low driving speeds/RPM’s. This is everything under the carb. It is highest at idle and lowest at W.O.T.

Checking Vacuum Type

The easiest way to check vacuum ports and telling what type of vacuum they are drawing is to start the engine with the vacuum ports capped off and uncap them one at a time. Once you uncap the port, touch the opening with your finger (at idle) if it sucks on your finger tip it is probably manifold vacuum. Increase to full throttle, the suction should dissipate. If the vacuum is ported, it should have little to no suction at idle and should start to draw respectable suction as acceleration is increased. Ported vacuum will reach peak suction at W.O.T

The above description is very incorrect. Manifold vacuum is high at idle and should increase as the throttle is opened with the engine under no load (as in neutral). This increase in vacuum will start to drop as the throttle is opened farther. Seeing the increase in vacuum with rpm increase is a good test of an unrestricted exhaust. With the engine under slight load manifold vacuum will be slightly lower as in cruising down the highway. Heavy load almost no vacuum to zero. The statement above about manifold vacuum retarding timing is rediculous.

Port vacuum will be zero at idle, unless idle speed is too high. Port vacuum will act almost exactly as manifold vacuum, increases with throttle opening under no load. Port vacuum will drop under load, and should be close to no vacuum at WOT. You do not want vacuum advance at WOT or under heavy load. This will cause detonation. If vacuum was that linear as stated above why would you need it to advance the timing. The mechanical advance would do the same thing. Another item that is hooked up to port vacuum in EGR, We don't want EGR opening at idle, perfect function for port vacuum, we also don't want the EGR opening at WOT when we need the extra power. Also look at racing distributers, they don't have vacuum advances (at least the ones I remember), know why? No vacuum at wide open throttle.

I hope this helps, and anyone who doesn't believe me hook up a vacuum gauge and drive, test one at manifold vacuum and one at port vacuum. I've been doing this for almost 30 years and the vacuum gauge used to be my favorite diagnostic tool.

algemech,
I am not dissagreeing with you in any way but if that is the case then why do we have vacuum advance on the distributer if the timing did not need to be advanced as RPM's get higher? I definetly want to know more about this.

algemech,
I am not dissagreeing with you in any way but if that is the case then why do we have vacuum advance on the distributer if the timing did not need to be advanced as RPM's get higher? I definetly want to know more about this.

Timing does need to be advanced as RPM's increase, but when you get near WOT and/or heavy load, the vacuum drops off. The mechanical advance is only reliant on RPM.

I agree with algmmech with everything except the part about the vacuum increase at the manifold port as the throttle just starts to open, then the vacuum decreases when throttle is increased. But then again, I drive with a heavy foot and have no reason not to believe him. I do a lot of tuning with a vac gauge as well.

This is BS. Vacuum is vacuum, the level of vacuum and how it plays out in relation to the throttle is the only thing different. There are people that run manifold a vacuum source instead of ported to the dizzy and have no ill effects. Manifold vacuum will not "retard your timing", you just will have a different amount of spark advance. I've done it and to be honest, didn't notice much in the way of performance gain or loss as a result. Some engine performance people will specifically tell you to run manifold vacuum. Try it.

You can look at it like this. The manifold port reads it's maximum at idle and will decrease under load or throttle opening at a somewhat proportionate rate (as long as load remains constant) until it reads next to nothing at WOT. Ported vacuum will read zero at idle (or it should unless the idle is too high). When the vacuum plates open but not a lot of load (read higher RPM with only partially open throttle plates), vacuum becomes present and is at a fairly substantial level. When load increase (read low RPM, open throttle plates) or the RPM's increase with no load, the vacuum level starts to drop off. At WOT, again, near zero vacuum.

When plotted out, the manifold source will start off at 100% and slowly taper to zero, fairly linear and predictable. The ported source will start at zero, rise to it's peak under partial throttle and no load, the taper back off again and will look more like a bell curve.

OK, basically the manifold goes down as throttle goes up and I am assuming the ported hits the peak of the "bell curve" when the engine makes peak Hp then drops off as the Hp drops off at higher RPM's.
Is that about the gist of it?